Tachyarrhythmias are abnormal heart rhythms characterized by a rapid rate, typically expressed in units of beats per minute (bpm), that can originate in either the ventricles or the atria. Examples of tachyarrhythmias include sinus tachycardia, atrial tachycardia, atrial fibrillation, ventricular tachycardia, and ventricular fibrillation. The most dangerous tachyarrythmias are those that have their origin in the ventricles, namely ventricular tachycardia (VT) and ventricular fibrillation (VF). Ventricular rhythms occur when re-entry of a depolarizing wavefront in areas of the ventricular myocardium with different conduction characteristics becomes self-sustaining or when an excitatory focus in the ventricle usurps control of the heart rate from the normal physiological pacemaker of the heart, the sino-atrial node. The result is rapid contraction of the ventricles out of electromechanical synchrony with the atria. Most ventricular rhythms exhibit an abnormal QRS complex in an electrocardiogram (ECG) because they do not use the specialized conduction system of the ventricles, the depolarization spreading instead from the excitatory focus or point of re-entry directly into the myocardium. In ventricular tachycardia, the ventricles contract rapidly and produce distorted QRS complexes in an ECG. Ventricular fibrillation, on the other hand, occurs when the ventricles depolarize at an even more rapid rate and in a chaotic fashion, resulting in QRS complexes of constantly changing shape and virtually no effective pumping action. Both ventricular tachycardia and ventricular fibrillation are hemodynamically compromising, and both can be life-threatening. Ventricular fibrillation, however, causes circulatory arrest within seconds and is the most common cause of sudden cardiac death.
Cardiac rhythm management devices known as implantable cardioverter/defibrillators (ICDs) are designed to treat ventricular tachyarrhythmias by delivering an electrical shock pulse to the heart. Cardioversion and/or defibrillation can be used to terminate most tachyarrhythmias, including VT and VF. The electric shock terminates the tachyarrhythmia by depolarizing all of the myocardium simultaneously and rendering it refractory.
Another type of electrical therapy for tachycardia is antitachycardia pacing (ATP). In ATP, the heart is competitively paced with one or more pacing pulses in an effort to interrupt the reentrant circuit causing the tachycardia. ATP therapy can successfully treat VT, but it is not effective in terminating VF. Modem ICDs incorporate ATP capability so that ATP therapy can be delivered to the heart when a tachycardia is detected. Although cardioversion/defibrillation will terminate tachycardia, it consumes a large amount of stored power from the battery and results in patient discomfort owing to the high voltage of the shock pulses. It is desirable, therefore, for the ICD to use ATP to terminate a tachyarrhythmia whenever possible.
In most ICDs with ATP capability, VF is distinguished from VT using rate-based criteria so that ATP or shock therapy can be delivered as appropriate. The heart rate is usually determined by measurement of the time interval between successive ventricular depolarizations. A measured heart rate is classified as a tachycardia when the rate is in a VT zone, defined as a range of rates above a tachycardia detection rate (TDR) but below a fibrillation detection rate (FDR). A measured heart rate above the FDR, on the other hand, is in the VF zone and is classified as fibrillation. In a typical device, a tachyarrhythmia with a heart rate in the VT zone is treated with ATP therapy in order to avoid an unnecessary painful shock to the patient, and a defibrillation shock is delivered if the heart rate is in the VF zone or if ATP pacing fails to terminate a tachyarrhythmia in the VT zone.
As aforesaid, VT can be detected when the ventricular rate falls within the VT zone. A rapid ventricular rate in the VT zone, however, is not necessarily due to VT but can also result from a tachyarrhythmia that originates from above the ventricles. Such tachyarrhythmias are referred to as supraventricular tachycardias (SVT's) and include sinus tachycardia, atrial tachycardia, and atrial fibrillation. The normal rhythmic impulse of the heart is first generated in pacemaker tissue known as the sino-atrial (SA) node, spreads throughout the atria causing atrial contraction, and is then conducted to the atrioventricular (AV) node where the impulse is delayed before passing into the ventricles. The ventricles of a normal heart are then electrically stimulated by excitation emanating from the AV node that spreads via specialized conduction pathways. An abnormal rhythm in the atria can thus be transmitted antegradely to the ventricles in patient whose AV conduction pathway is intact. Such an SVT is characterized by elevated rates in both the atria and the ventricles. Elevated rates in both the atria and ventricles can also occur with VT as well, however, due to retrograde conduction of excitation from the ventricles to the atria. Such retrograde conduction is possible in most people and confounds the discrimination between VT and SVT based upon atrial and ventricular rates alone when both rates are similar, a condition known as a one-to-one or 1:1 tachycardia.
It is desirable for an ICD to differentiate between an SVT and a VT, however, both for reasons of efficacy and safety. ATP therapy delivered to treat an SVT will not be effective and potentially could make matters worse by triggering a ventricular arrhythmia. Also, although most ICD's currently on the market today are designed to treat only tachyarrhythmias of ventricular origin, some newer designs are capable of treating atrial tachyarrhythmias as well. It is thus important for an ICD to recognize that an elevated ventricular rate is due to an SVT rather than a VT so that either ventricular ATP therapy can be withheld or more appropriate therapy can be delivered. Conversely, because VT is generally a more serious condition, the ICD also needs to detect VT with a high degree of sensitivity so that therapy can be delivered promptly.